This invention relates to a glass-ceramic substrate for a magnetic information storage medium such as a magnetic disk and, more particularly, to a glass-ceramic substrate for a magnetic information storage medium having a super-flat substrate surface which is suitable for a near-contact recording system or a contact recording system and having also excellent surface characteristics in a landing zone of the substrate for preventing sticking of a magnetic head to a magnetic disk in the CSS (contact start and stop) recording system. The invention relates also to a magnetic information storage medium formed by subjecting the glass-ceramic substrate to a film forming process.
In the present specification, the term "magnetic information storage medium" denotes a disk-like magnetic information storege medium such as a hard disk used in a personal computer including a fixed type hard disk, a removal type hard disk and a card type hard disk and a disk-like magnetic information storage medium which can be used in a digital video camera or a digital camera.
There is an increasing tendency to utilizing a personal computer for multiple media purpose and using a digital video camera and a digital camera with resulting handling of a large amount of moving image and voice information and this tendency necessitates an information storage medium of a large recording capacity. For this purpose, the bit number and track density of a magnetic information storage medium must be increased and the size of a bit cell must be reduced for increasing a longitudinal recording density. As for a magnetic head, it must be operated in closer proximity to the surface of the magnetic information storage medium in conformity with the reduction of the bit cell size. In a case where a magnetic head is operated at an extremely low flying height (i.e., in a near-contact state) or in a contact state against a magnetic information storage medium, there are provided, as a technique for start and stop of a magnetic information storage device, (1) the landing zone system according to which a landing zone is provided for performing the CSS (contact start and stop) operation therein in a particular portion of a magnetic information storage medium substrate (mainly an unrecorded portion in an inner peripheral side of the magnetic information storage medium) which is subjected to a processing for preventing sticking of a magnetic head (texturing processing) and (2) the ramp loading system according to which a magnetic head is in a stand-by state in a position outside of an outer periphery of a magnetic information storage medium when the magnetic information storage medium is at a standstill and, when the magnetic information storage medium has started, the magnetic head is shifted to a position above the magnetic information storage medium after starting of its rotation and lowered softly on the magnetic information storage medium and, when the magnetic information storage medium is to be stopped, the magnetic head is lifted while the magnetic information storage medium is rotating and then the magnetic head is shifted to the position outside of the outer periphery of the magnetic information storage medium.
In the CSS system, if the surface of contact of the magnetic head with the magnetic information storage medium is formed to a mirror surface to an exceeding degree, sticking of the magnetic head occurs during stoppage of the medium with the result that rotation of the medium is not started smoothly due to increase in friction and damage to the surface of the medium occurs. In contrast, in the ramp loading system in which a magnetic head is located in a position outside of the outer periphery of a magnetic information storage medium when the medium is at a standstill and rotation of the medium is started and the magnetic head is loaded on the medium only when the medium is rotating, an accurate operation control for loading the magnetic head is required but the landing zone required in the CSS system becomes unnecessary, so that a portion of the medium which is allotted to the landing zone can be utilized as a data zone and, therefore, the recording capacity of the medium can be increased. Further, according to the ramp loading system, damage to the surface of the medium at the start of rotation of the medium can be avoided.
As described above, in a magnetic information storage medium, two systems have been developed to cope with the conflicting demands for inputting and outputting of magnetic signals by a magnetic head at a low flying height or in a contact state necessitated by increase in the recording capacity and prevention of sticking of the magnetic head to the magnetic information storage medium. In either case, the data zone of a magnetic information storage medium must have a flatter surface than before and therefore a substrate for the magnetic information storage medium must also have a flatter surface than before. Further, development of a magnetic information storage media including a removal type hard disk and a card type hard disk in contrast to the conventional fixed type hard disk is under way along with development of a digital video camera and a digital camera and, as a result, higher characteristics including higher mechanical strength are required for a substrate of a magnetic information storage medium.
Aluminum alloy has been conventionally used as a material of a magnetic information storage medium substrate. The aluminum alloy substrate, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process due to defects inherent in the material. As a result, the aluminum alloy substrate is not sufficient in flatness. Further, since an aluminum alloy is a soft material, deformation tends to take place so that it cannot cope with the recent requirement for making the magnetic information storage medium such as a magnetic disk thinner and the requirement for high density recording because the magnetic information storage medium tends to be deformed by contact with the magnetic head with resulting damage to the recorded contents.
As a material for overcoming this problem of the aluminum alloy substrate, known in the art are glass substate for magnetic information storage medium made of a chemically tempered glass such as a sodium lime glass (SiO.sub.2 --CaO--Na.sub.2 O) and alumino-silicate glass (SiO.sub.2 --Al.sub.2 O.sub.3 --Na.sub.2 O). These glass substrates, however, have the following disadvantages:
(1) Polishing is made after chemical tempering and so the tempered layer tends to cause instability in thinning the magnetic information storage medium. PA0 (2) For improving the CSS characteristics, the substrate must be subjected to texturing which produces projections and depressions on the surface of the substrate. Since a mechanical processing or a thermal processing such as by laser beam causes a cracking or other defects due to distortion in the chemically tempered layer, texturing must be conducted by a chemical etching or sputtering but this prevents a stable production of the product at a competitive cost. PA0 (3) Since the Na.sub.2 O ingredient is included as an essential ingredient in the glass, the magnetic thin film characteristics of the glass are adversely affected with the result that barrier coating over the entire surface of the glass becomes necessary for preventing elution of Na.sub.2 O ingredient and this prevents stable production of the product at a competitive cost. PA0 (1) A carbon substrate is formed by pressing at a high pressure and burning at a high temperature of about 2600.degree. C. with resulting difficulty in a large scale production at a low cost. PA0 (2) A carbon substrate is low in its mechanical properties (Young's modulus and fracture strength) so that it is difficult to cope with the thinning tendency and a high speed rotation of a drive. PA0 (3) The forming of the landing zone utilizes oxidation and evaporation of carbon by the pulsed laser. Since carbon is a material which causes a very strong thermal oxidation reaction, the formed landing zone becomes instable and thus it poses a serious problem in reproduceability.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are some substrates made of glass-ceramics. For example, Japanese Patent Application Laid-open No. 6-329440 discloses a glass-ceramic of a SiO.sub.2 --Li.sub.2 O--MgO--P.sub.2 O.sub.5 system which includes lithium disilicate (Li.sub.2 O.2SiO.sub.2) and .alpha.-quartz (.alpha.-SiO.sub.2) as predominant crystal phases. This glass-ceramic is an excellent material in that, by controlling the grain size of globular grains of .alpha.-quartz, the conventional mechanical texturing or chemical texturing becomes unnecessary and surface roughness (Ra) of a polished surface can be controlled within a range from 15 .ANG. to 50 .ANG.. This glass-ceramic cannot cope with the above described target surface roughness (Ra) of 1 .ANG. to 5 .ANG. and also cannot sufficiently cope with the above described tendency to lowering the flying height of a magnetic head necessitated by rapid increase in the recording capacity. Besides, in this glass-ceramic, no discussion about the landing zone to be described later in this specification is made at all.
Japanese Patent Application Laid-open No. 7-169048 discloses a photo-sensitive glass-ceramic of a SiO.sub.2 --Li.sub.2 O system including Au and Ag as photo-sensitive metals characterized in that a data zone and a landing zone are formed on the surface of a magnetic disk substrate. A predominant crystal phase of this glass-ceramic is lithium silicate (Li.sub.2 O.SiO.sub.2) and/or lithium disilicate (Li.sub.2 O 2SiO.sub.2). In case lithium silicate is used, the glass-ceramic has a poor chemical durability so that it has a serious practical problem. Further, in forming of the landing zone, a part of the substrate (i.e., landing zone) is crystallized and is subjected to chemical etching by using 6% solution of HF. However, forming of the substrate with an uncrystallized part and a crystallized part makes the substrate instable mechanically as well as chemically. As for chemical etching by HF solution, it is difficult to control concentration of the HF solution because of evaporation and other reasons so that this method is not suitable for a large scale production of products.
Japanese Patent Application Laid-open No. 9-35234 discloses a magnetic disk substrate of a SiO.sub.2 --Al.sub.2 O.sub.3 --Li.sub.2 O system in which predominant crystal phases consist of lithium disilicate (Li.sub.2 O.2SiO.sub.2 O) and .beta.-spodumene (Li.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2). The predominant crystal phase of this glass-ceramic is .beta.-spodumene which has a negative thermal expansion characteristic (as a result, the substrate has a low thermal expansion characteristic) and growth of crystal phases of the SiO.sub.2 system such as .alpha.-quartz (.alpha.-SiO.sub.2) and .alpha.-cristobalite (.alpha.-SiO.sub.2) which have a positive thermal expansion characteristic (as a result, the substrate has a high thermal expansion characteristic) is restricted. It is described that this glass-ceramic provides a surface roughness (Ra) after polishing of 20 .ANG. or below but surface roughness (Ra) disclosed in Examples is 12 .ANG. to 17 .ANG. which is not sufficient for the above described demand for a super flat surface and cannot cope with the tendency to a low flying height of a magnetic head necessitated by increase in the recording capacity. This glass-ceramic contains Al.sub.2 O.sub.3 ingredient which is indispensable for growing of .beta.-spodumene in the amount exceeding 5%. The material in which the crystal having a negative thermal expansion characteristic grows as a predominant crystal phase apparently produces an adverse effect in respect of difference in the coefficient of thermal expansion between the magnetic information storage medium and components of the information storage medium device. Besides, this glass-ceramic requires a high temperature of 820.degree. C.-920.degree. C. for crystallization and this prevents a large scale production of the product at a low cost.
International Publication WO97/01164 includes the above described Japanese Patent Application Laid-open No. 9-35234 and discloses a glass-ceramic for a magnetic disk in which the lower limit of Al.sub.2 O.sub.3 ingredient in the above described composition system is lowered and the temperature for crystallization is lowered to 680.degree. C.-770.degree. C. The improvements of this glass-ceramic however are still insufficient in that the crystal phase of glass-ceramics of all examples is .beta.-eucryptite (Li.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2) which has a negative thermal expansion characteristic and therefore produces an adverse effect in respect of difference in the coefficient of thermal expansion between the magnetic information storage medium and components of the magnetic information storage medium device. In addition, these publications are characterized in that they do not substantially contain MgO ingredient.
Several methods are known for forming a landing zone and a data zone on the surface of a magnetic information storage medium. For example, Japanese Patent Application Laid-open No. 6-290452 discloses a method for forming a landing zone on a carbon substrate by a pulsed laser having a wavelength of 523 nm. In this case, however, there are the following problems:
Japanese Patent Application Laid-open No. 7-65359 and U.S. Pat. No. 5,062,021 disclose a method for forming a landing zone on an aluminum alloy by a pulsed laser. The aluminum alloy has the above described problems. Besides, the surface of the substrate after irradiation of laser beam tends to have a defect due to oxidation of a molten portion and remaining of splash of molten metal on the surface. It is therefore difficult to put this method to a practical use.
It is, therefore, an object of the invention to eliminate the above described disadvantages of the prior art and provide a glass-ceramic substrate for a magnetic information storage medium having excellent surface characteristics capable of realizing a stable flying height of a magnetic head in a landing zone where the contact start and stop of the magnetic head is performed and realizing inputting and outputting of magnetic signals at a low flying height or in a contact state of the magnetic head in a data zone (including the ramp loading system) satisfying the requirement for high recording density.
It is another object of the invention to provide a magnetic information storage medium made by forming a film of a magnetic medium on this glass-ceramic substrate.